The present invention relates in general to power supplies. More specifically, the present invention relates to current sharing in multiphase switching power supplies.
As microprocessors and their supporting components become larger and more complex, the load presented by these devices becomes heavier. As a consequence, the power supply, which powers the microprocessor and its supporting components, must be capable of providing higher currents.
One way of increasing the current capability of a power supply is to use two or more phases or multi-phase switching mode converters to drive the load. Typically, these multi-phase supplies are designed with phase interleaving in order to reduce ripple in the converted signal.
A recognized problem associated with multi-phase power supplies relates to an uneven distribution of load currents carried among the various phases of the supply. If this uneven distribution is substantial, a single phase may be burdened with supplying most of the load current. This problem not only limits the output capability of the supply, it also affects its reliability. Accordingly, methods of equalizing the distribution of load current among phases have been sought and developed. These methods have often been referred to in the art as xe2x80x9ccurrent sharingxe2x80x9d methods.
One xe2x80x9ccurrent sharingxe2x80x9d approach measures the voltage dropped across precision power resistors placed in the load current path of each of the power supply phases. This approach is undesirable for at least two reasons. First, use of precision power resistors is costly, especially as current demands continue to rise, since the resistors must be made physically larger. Second, because precision power resistors dissipate wasted power, the efficiency of the power supply is compromised.
To avoid reliance on highly dissipative and costly precision resistors, an alternative current sharing approach measures the voltage drop across switching elements in one or more phases of the supply. Because these voltage drops are proportional to the currents supplied by the corresponding power phases, they can be used to direct current sharing among power stages. Whereas this approach may be an improvement, in that it does not require use of precision power resistors, a problem is that it is not a very accurate method. For example, individual phase currents, derived from the measured voltages, can produce inaccuracies of up to 60%.
Accordingly, there exists a need for a simple, inexpensive and accurate current sharing method, which can be used in multi-phase power supplies.
According to a first aspect of the invention, a current sharing multi-phase power supply comprises a first phase unit, having a first package containing a first dc-to-dc converter. The dc-to-dc converter of the first phase unit supplies a first portion of a load current to a load. The first phase unit further includes a first pulse width modulator coupled to the first converter. This first pulse width modulator functions to provide a first periodic pulse train to the first converter. The multi-phase supply also has at least a second phase unit, which includes a second dc-to-dc converter and a second pulse width modulator coupled to the second converter. The second pulse width modulator functions to provide a second periodic pulse train to the second converter and the second dc-to-dc converter supplies a second and remaining portion of the load current to the load. The supply further comprises a temperature control bus, which is coupled to the first and second phase units. The temperature control bus has a bus temperature related to temperatures of the first and second packages. Duty cycles of the first and second pulse trains are adjusted in a feedback operation, the extent of the adjustments depending on a relationship among the temperatures of the first and second packages and the bus temperature. Depending on the application, the bus voltage on the temperature control bus represents either an average or a peak of the measured temperatures of the first and second packages.
In a second aspect of the invention, a multi-phase power supply having a plurality of phase blocks is disclosed. Each phase block comprises a package containing a dc-to-dc converter, a temperature sensor and a temperature-to-voltage converter. Each phase block also includes a pulse width modulator disposed within a feedback loop and coupled between an output of the temperature-to-voltage converter and an input of the converter.
In a third aspect of the invention, a method of promoting current sharing among phases of a multi-phase power supply comprises the steps of, measuring a temperature of a package containing a dc-to-dc converter for each phase and, for each phase, employing a feedback control means, disposed between an output of the supply and a control input of the converter, to control and reduce the temperature variation and current variation among packages.